High frequency electron discharge device having improved permanent magnetic focusing



Nov. 1, 1966 0 E. PALLAKOFF 3,283,200 HIGH FREQUENCY ELECTRON DISCHARGE DEVICE HAVING IMPROVED PERMANENT MAGNETIC FOCUSING Filed Dec. 12, 1963 FIG.I

PRIOR ART 5 e GI 'I FIG. 6

AXIAL FIELD ALONG PACKAGE CENTERLINE AXIAL FIELD IN MEDIAN PLANE SHIELDED MAGNET 2 3 4 5 6 7 DISTANCE FROM PACKAGE (INCHES) FIG.5

INVENTOR OWEN E. PALLAKOFF BY k ATTORNFY United States Patent Ofiice 3,2332% Patented Nov. 1, 1966 3,283,200 HIGH FREQUENCY ELECTRON DISCHARGE DE- VICE HAVING IMPROVED PERMANENT MAG- NETIC FOCUSHNG Owen E. Pallakolf, Los Altos, Calif., assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed Dec. 12, 1963, Ser. No. 330,029 1 Claim. (Cl. 315-35) This invention relates in general to magnetic focusing techniques for linear beam electron discharge devices and more particularly to novel permanent magnetic focusing means coupled with flux return shields for providing novel shielded permanent magnetic focusing structures for linear beam electron discharge devices.

US. patent application, Serial No. 330,728, by William L. Beaver, filed December 16, 1963, assigned to the same .assignee as the present invention, points out certain deficiencies in prior art magnetic focusing schemes for linear beam discharge devices and discloses a novel permanent magnet focusing technique as an improvement over such prior are devices. This invention pertains to improvements in the aforementioned application. Specifically, this invention relates to optimizing and increasing the main unidirectional axially symmetric magnetic focusing field used in a linear beam electron discharge device. The aforementioned copending US application by William L. Beaver discloses a broad focusing concept for linear beam discharge devices.

The present invention enhances or increases the main focusing field through the linear beam device without increasing the overall size of the focusing package. This is accomplished through the novel concept of adding axially polarized auxiliary magnets to the main focusing structure intermediate the ends of the main focusing structure. These axially polarized auxiliary magnets, when oriented correctly will add to the main focusing field provided by the magnets disposed at opposite ends of the device.

In another embodiment of the present invention the field strength available along the longitudinal axis of the linear beam electron discharge device is increased by positioning within a given sized focusing package two hemispherically shaped axially aligned permanent magnets substantially around the electron discharge device. The maximum unidirectional magnetic focusing field is achieved for a given volume through the utilization of this concept.

Therefore, it is an object of this invention to increase the unidirectional magnetic focusing field along the longitudinal axis of a traveling wave tube by the utilization of axially polarized auxiliary magnets in combination with a main focusing structure including axially polarized magnets disposed at opposite ends of the tube in conjunction with a surrounding flux guide.

A feature of the present invention is the provision of a linear beam electron discharge device having a pair of axially polarized auxiliary permanent magnets spaced along the longitudinal axis of said device intermediate of and cooperating with a pair of axially polarized permanent magnets disposed at each end of said electron discharge device.

Another feature of the present invention is the provision of a novel magnetic focusing package capable of providing the characteristics set forth in the aforementioned features.

These and other features of the present invention will become more apparent upon a perusal of thefollowing specification taken in conjunction with the accompanying drawings wherein:

.the pole pieces as shown.

FIG. 1 is a schematic view, partially sectioned, of a typical prior art bowl type permanent magnet focusing technique utilized to provide focusing fields for linear beam electron discharge devices;

FIG. 2 is a cross-sectional view partly in elevation depicting a backward wave oscillator type traveling wave tube having a unidirectional focusing field provided by the novel permanent magnet focusing techniques of the present invention;

FIG. 3 is a cross-sectional view taken along the lines 33 of FIG. 2 depicting the cross-section of the focusing structure and tube shown in FIG. 2;

FIG. 4 is a schematic view partly sectioned depicting one embodiment of the novel focusing technique employed in the present invention;

FIG. 5 is an alternative embodiment depicting the novel hemispherically shaped permanent magnets utilized for providing a unidirectional main focusing field; and

FIG. 6 is a graphical portrayal comparing the exterior leakage fields present in the prior art bowl type focusing structure of the type shown in FIG. 1 with the exterior leakage fields present in the novel shielded focusing structure such as depicted in FIG. 2.

Referring now in particular to FIG. 1 there is depicted .a linear beam focusing structure of the bowl type including a pair of semi-cylindrically shaped permanent magnets 1 and 2 paralleling a linear beam electron discharge device 3 which in this case can be a backward wave oscillator (BWO) such as depicted in FIG. 2. Magnets 1 and 2 are coupled to pole caps 4, 4', e.'g., soft iron, through annular pole pieces 5, 5', e.g., soft iron having suitable apertures for the necessary electrical conductors required for tube operation. The dotted lines are representative of the high degree of external flux leakage present in a focusing structure such as shown in FIG. 1. Useful flux extends between pole caps 4, 4' along the centerline 6 while all other external leakage flux is wasted.

In FIG. 2 there is shown a representative linear beam electron discharge device 7 incorporating the novel focusing assembly 8 as taught by the present invention. The electron discharge device 7 shown in FIG. 2 is a BWO. For the details of a BWO such as that depicted in FIGS. 1-5 see, for example, US. Patent No. 2,991,391 by William L. Beaver. In brief, the electron discharge device 7 is supported within a pair of annular pole pieces 9 and 1t). Teflon shock absorbing annular support sleeves 11 and 12 are disposed in annular recesses within The output RF. energy of the backward wave oscillator 7 is taken from output waveguide 15 through suitable apertures provided in exterior flux guide 16. The flux guide 16 is preferably two annular cup shaped members such as disclosed in the aforementioned copending application Serial No. 330,728 by William L. Beaver, filed December 16, 1963. The main undirectional axially symmetric focusing field for backward Wave oscillator 7 is provided by a series of axially polarized permanent magnets 17, 18 having a ring like annular form. These magnets, as taught by the aforementioned copending application by W. L. Beaver are disposed as shown at the opposite ends of the backward wave oscillator. The axially polarized magnets 17 and 18 may either be a plurality of annular rings as shown in FIG. 1 or alternatively they may take the form of annular discs having suitable apertures axially directed therethrough as taught by the aforementioned application of W. L. Beaver. It is to be noted that in either case the flux return guide 16 completely surrounds and encloses the permanent magnet focusing assembly thereby providing a flux shield and guide wherein all points on the flux guide are at approximately the same magnetic potential. Furthermore, it is to be noted that cember 16, 1963, in the following respects.

rectangular, square or other shapes beside annular are contemplated by the present invention.

In order to maximize the unidirectional main focusing field the present invention provides a novel concept of parallel axially polarized auxiliary permanent magnets 19 and 20. The magnets are preferably'oriented as shown. It is to be seen that the polarization of the auxiliary magnets is such that the fields produced by the auxiliary magnets are additive with the main field produced by the magnets 17, 18 as shown. It is readily seen that a substantial increase in total unidirectional main focusing field is obtained by the novel paralleled focusing concept taught by the present invention.

As more clearly shown in FIG. 3 the present invention includes a pair of transversely spaced segmented shaped permanent magnets 21 and 22. The permanent magnets 21 and 22 provide additional magnetomotive force which is coupled to the main focusing field. Suitable apertures are provided in the flux guide 16 for the power and RF. conductors for the vBWO. The apparatus depicted in FIG. 1 is essentially completed with the addition of a pair of aluminum or the like annular adjustable nonmagnetic shim members 23, 24. Each of said aluminum shim members 23, 24 is provided with threaded apertures wherein adjusting set screws 25' are positioned to bear against the exterior periphery of flanges 26, 27 of pole piece members and 11. Said screws 25 are advantageously utilized to shift the pole pieces 9 and 10 thereby moving the electron discharge device to enable the optimum focusing position to be achieved by alignment of the axes of the electron beam and the focusing field. Field correcting shims 28, 29 are advantageously employed to optimize the main focusing field as taught in the aforementioned U .S. Patent No. 2,991,391.

Refer-rng now to FIG. 4-, a schematic view of the novel focusing techniques of the present invention is depicted therein. FIG. 4 is an improvement over the aforementioned W. L. Beaver U.S. patent application filed De- As can be seen .a single axially polarized perfanent magnet 30 having a suitable aperture for RF. conductors surrounds the electron discharge device 31. This enable the main unidirectional focusing field extending along the longitudinal axis of the electron discharge device 31 to be substantially increased without increasing the overall focusing volume. The important feature depicted therein is that the axial polarization of the auxiliary magnet 30 is reversed with respect to the axial polarization of the end magnets, 32, 33. This simple but novel concept enables extremely strong focusing fields to be obtained for a given overall focusing apparatus volume. It is to be noted that flux guide 34 having a suitable aperture for RF. conductors surrounds the permanent magnet focusing assembly and function as a fiux guide by providing a -low reluctance magnetically shorted path between end magnets 32, 33 and magnetic shield for the focusing magnets and the linear beam electron discharge device in the same fashion as taught in the aforementioned copending application by W. L. Beaver.

FIG. 5 depicts an alternative embodiment wherein a pair of hemispherically shaped partially hollowed out permanent magnets are utilized to provide the main focusing field for an electron discharge device 37. The magnetic polarization utilized in the hemispheres 35 and 36 is radial as shown by the polarization arrows 39 thereby defining a pair of interior poles 41, 42 and a pair of exterior poles 43, 44. This novel scheme maximizes the unidirectional focusing field available for a given volume of magnetic material. It is apparent that since the exterior periphery of each of the hemispheres .35 and 36 comprises a single north or south pole that the flux guide 38 can intimately contact the exterior of the hemisphere without shorting the poles of one of the hemispheres. Therefore, all of the advantages in FIG. 3 are obtained with 1th? scheme of FIG. 4 with the additional advantage of maximizing the total unidirectional main focusing field for a given focusing apparatus volume.

A backward wave oscillator operable in the frequency range of 82-124 megacycles and constructed in general as taught by the aforementioned U.S. Patent 2,991,391 by W. L. Beaver was positioned within the focusing assembly depicted in FIG. 2 as shown in FIG. 2 and operated successfully. The external leakage field /2" from the outer periphery of the flux shield 16 was less than 25 gauss. Measurements with regard to extraneous magnetic fields and their possible degradation of the main focusing field along the longitudinal axis of the backward wave oscillator were found to be practically nonexistent when using focusing techniques depicted in FIG. 1. FIG. 6 is a graphical comparison of typical external leakage fields present in a BWO utilizing the focusing technique of the prior art as exemplified by FIG. 1 and the focusing techniques of the present invention as exemplified by FIG. 2. Characteristic A is representative of the external or stray axially directed magnetic field in oersteds along the center line 6 of FIG. 1 as measured outwardly from the external surface of the focusing structure along the center line or axis 6 thereof. Characteristic *B is representative of the external or stray axially directed magnetic field in oersteds along the medium plane 6 of FIG. 1 as measured outwardly from the external surface of the focusing structure along the medium plant 6' thereof. Characteristic C is representative of the external or stray axially directed magnetic field in oersteds along the center line or axis of FIG. 2 as measured outwardly from the external surface of the focusing structure along the center line or axis thereof. External or stray flux along the medium plane M of FIG. 2 was inconsequential since it was practically speaking nonexistent; therefore, no characteristic of same is given. The above characteristics speak for themselves with regard to the order of improvement resulting through the utilization of the teachings of the present invention. The characteristics of FIG. 6 were obtained with Alnico V magnet materials in the case of the bowl magnets and Indox V in the case of the shielded magnets of the present invention. In each case the magnets were producing a useful gap flux of approximately 1400 oersteds. The physical size of focusing structure of FIG. 2 will be smaller than that of FIG. 1 for a given useful gap flux. Obviously the characteristics depicted in FIG. 6 with regard to leakage flux of the shielded magnets of FIG. 2 are applicable to the shielded versions depicted in FIGS. 4 and 5. It is to be understood that the focusing techniques described herein may advantageously be employed with other electron discharge devices such as linear beam traveling wave tube amplifiers and linear beam klystron tubes. Examples of such devices are to be found in U.S. Patent No. 3,051,866 by R. B. Nelson, U.S. Patent No. 3,103,609 by L. T. Zitelli and U.S. Patent No. 2,836,758 by M. Chodorow. It is further understood that the tubes in FIGS. 2-5 are also representative of traveling wave tube amplifiers and klystrons such as shown in the aforementioned patents although panticularly drawn to a BWO such as shown in U.S. Patent No. 2,991,391.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

An apparatus comprising a linear beam high frequency electron discharge device including vacuum envelope means within which electron beam forming and projecting means are disposed at the upstream end portion of said device for generating and projecting an electron beam along an elongated longitudinal axis along which means for providing an energy exchange between said electron beam and electromagnetic wave energy are disposed between the upstream and downstream end portions of said device, said apparatus including at least a pair of spatially displaced and axially polarized permanent magnets disposed at the respective opposite ends of said linear beam high frequency electron discharge device externally of said vacuum envelope means, said one of said pair of permanent magnets having inner and outer poles of opposite magnetic polarity than the polarity of the inner and outer poles of said other of said pair of permanent magnets, the axis of polarization of said pair of permanent magnets being substantially aligned with the longitudinal axis of said electron discharge device, said pair of permanent magnets providing a unidirectional magnetic field between the upstream and downstream end portions of said electron discharge device for focusing said electron beam, said apparatus including means forming a flux guide and shield substantially surrounding said linear beam electron discharge device and said pair of permanent magnets, said flux guide and shield being made of a high permeability material, said flux guide and shield being physically related to said pair of permanent magnets and said electron discharge device such that said flux guide and shield provide a low reluctance magnetic circuit between the outer axially oriented poles of said pair of permanent magnets and such that said flux guide and shield provide a magnetic shield for said electron discharge device and said permanent magnets, said flux guide and shield including a hollow axially elongated shell disposed about and transversely spaced from said high frequency electron discharge device, said flux guide and shield including a pair of transverse end walls magnetically shorted to the respective outer poles of said pair of permanent magnets, said vacuum envelope including said upstream and downstream end portions of said electron discharge device being disposed entirely internally of the transverse end walls of said flux guide and shield, the exterior transverse dimensions of said permanent magnets being smaller than the interior transverse dimensions of said hollow shell forming said flux guide and shield, the transverse opposing wall portions of said permanent magnets and said surrounding hollow shell being physically spaced from each other, said apparatus further including a pair of high permeability pole caps disposed at opposite ends of said electron discharge device externally of said vacuum envelope, said pole caps being disposed on the respective inner poles of said permanent magnets, said apparatus further including at least a pair of auxiliary permanent magnets disposed intermediate said at least one pair of permanent magnets disposed at the respective end portions of said electron discharge device, the polarization of said auxiliary magnets being opposite the polarization of said permanent magnets disposed at the respective end portions of said high frequency electron discharge device, said apparatus further including a pair of transversely spaced segmental shaped permanent magnets positioned along the longitudinal axis of said device intermediate said auxiliary permanent magnets, said segmental shaped permanent magnets being polarized in the same direction as the auxiliary magnets said apparatus further including annular hollow spacer members made of nonmagnetic material disposed exteriorly of said auxiliary permanent mag nets and interiorly of said flux guide and shield and in intimate contact with both said auxiliary permanent magnets and said flux guide and shield shell member.

References Cited by the Examiner UNITED STATES PATENTS 2,934,666 4/1960 Shrader 3l384 2,936,394 5/1960 Brewer 3153.5 2,956,193 10/1960 De Wit 3l53.5 X 3,134,925 5/1964 Buck 315-3.5 3,182,234 5/1965 Meyerer 3153.5 X 3,205,415 9/1965 Seki et a1 317-200 FOREIGN PATENTS 1,136,020 9/ 1962 Germany.

HERMAN KA-RL SAALBACH, Primary Examiner, ELI LIEBERMAN, Examiner. S, C I-TATMON, IR., Assistant Examiner, 

